The present invention is directed to optical communications. More particularly, the present invention is directed to an optical wavelength converter.
Optical communication systems increasingly utilize Dense Wavelength Division Multiplexing (xe2x80x9cDWDMxe2x80x9d) to increase the available bandwidth in installed optical fiber. DWDM involves the transferring of information in the form of different wavelengths or channels on the same fiber. In a DWDM system, individual bitstreams modulated on each channel can be redirected to other fibers at each node through the use of add/drop filters.
In very small optical networks, particular wavelengths can be dynamically allocated to individual connections, and it is possible to route a signal from end-to-end across the network without having to change wavelengths. However, when such a network is scaled to a reasonably large size, allocation of unique end-to-end wavelengths becomes very difficult to achieve. The network could have many vacant optical channels or wavelengths over all of its links, but a single unique wavelength may not be available on any of the possible paths between two end users. To overcome this problem, it is necessary to change the wavelength of some signals as they traverse the network by using a wavelength converter.
An ideal wavelength converter is a single input/output device that converts the wavelength of a channel appearing on its input port to a different value at its output port, but otherwise leaves the optical signal unchanged. Wavelength converters may be separated into two categories: those based on optical-gated effects, and those that use wavelength mixing, referred to as xe2x80x9ccoherent convertersxe2x80x9d. Optically gated converters typically operate on a single input signal and are not transparent to bit rate and modulation format, whereas the wavelength mixing converters operate transparently on multiple signals within a broad band of wavelengths. Within this latter category, devices based on four-wave mixing and difference frequency generation (also referred to as xe2x80x9cthree-wave mixingxe2x80x9d) are known.
The most commonly used wavelength converter is an opto-electronic wavelength converter which first converts the optical signal into electrical form before converting it to a different wavelength. In an opto-electronic wavelength converter, an intensity modulated signal at wavelength xcex1 is converted to electrical form in a photodetector, amplified, and used to modulate a laser operating at a different wavelength xcex2. Although an opto-electronic wavelength converter is good in terms of power output or gain, it is inherently nonlinear, and hence opaque. For example, two superimposed signals at different wavelengths cannot be converted simultaneously. In addition, an opto-electronic wavelength converter, because it requires electronic components, consumes a lot of power, adds significant noise to the signal, and is expensive to design for very high bit rates because of the problem with cross-talk.
Another known optical converter is a device that uses cross-talk in a Semiconductor Optical Amplifier (xe2x80x9cSOAxe2x80x9d). SOAs have severe cross-talk when operating close to saturation. When a relatively high level signal is fed into an SOA, it saturates. Specifically, the gain medium gives up all, or nearly all, of its excited state electrons and for a short time until more energy is supplied by the pump it cannot amplify any more. If two DWDM signals are fed to an SOA at saturation, the result is very severe cross-talk between them for the above reason.
Optical converter devices make intentional use of this cross-talk by feeding a modulated signal at a relatively high intensity to the SOA. This is mixed with another lower intensity unmodulated signal at a different wavelength, referred to as the xe2x80x9cprobexe2x80x9d. On exit from the SOA, the probe signal will now carry the modulations from the original data signal. However, the modulations are the inverse of the unmodulated signal. The original signal is then filtered out. One problem with this type of wavelength conversion is that a high enough signal level can only be achieved by using a relatively high-level data signal and a low level probe signal. This means that the data signal needs to be preamplified before entering the SOA, and the probe needs to be post-amplified at the output.
Another known method of optical conversion is the use of cross-phase modulation in an SOA. Operating at saturation intensity modulation in one signal stream can affect the refractive index of the active region in an SOA. This changes the phase of all signals passing through it. In optical conversion devices, the changes in phase are converted to changes in amplitude by situating the SOA in one arm of a Mach-Zender interferometer.
Four-wave mixing (xe2x80x9cFWMxe2x80x9d) in an SOA is another known method of optical conversion. In the phenomenon of four-wave mixing, an unmodulated probe signal is mixed with the original signal. FWM utilizes the cross talk incurred by passing a high amplitude optical signal through a material with a large cubic nonlinearity. The wavelength of the probe signal is chosen so that one of the sideband signals produced will have the desired wavelength. Various ways are used to separate the desired signal from the input signal and the probe such as by using a circulator and an in-fiber Bragg grating.
FWM can operate at high bit rates and is modulation format independent. FWM produces two output frequencies, one of which must be filtered out. The probe signal in FWM is close to the wavelength of the input and output signals. One problem with FWM is that the wavelength of the output signal is a function of the wavelength of the input signal, so unwanted variations in the input signal create similar wavelength variations in the output.
Wavelength conversion using Difference Frequency Generation (xe2x80x9cDFGxe2x80x9d) is similar to FWM, except that DFG is a non-linear effect experienced within waveguides at relatively high power levels. DFG provides a very low noise operation and can shift multiple wavelengths at the same time. In addition, it""s fast and bi-directional. However, DFG is low in efficiency and very polarization sensitive.
Finally, acoustic filters and modulators have been used to shift the optical frequency by the amount of the acoustic frequency. However, to get any really significant shift in wavelength (e.g., 1 nm), a very high acoustic frequency (e.g., 130 GHz) is required. Such an acoustic frequency is not currently possible, but smaller wavelength shifts of approximately 1 GHz are currently realizable
Based on the foregoing, there is a need for an improved wavelength converter for converting the wavelength of optical signals.
One embodiment of the present invention is an optical wavelength converter that converts an optical input signal having a first wavelength into an optical output signal having a second wavelength. The wavelength converter includes a saturable absorber switch having a control beam waveguide and an input waveguide. The converter further includes a first input coupled to the control beam waveguide and adapted to receive the optical input signal, and a second input coupled to the input waveguide and adapted to receive a second optical signal having the second wavelength from an optical source.